Modern communications systems are ever more increasing in bandwidth, causing greater needs for broadband antennas Some may require a decade of bandwidth, e.g. 100-1000 MHz. Various needs (e.g. military needs) may require broadband antennas for low probability of intercept (LPI) transmissions or communications jamming. Jamming systems can use high power levels and the antenna must provide a low voltage standing wave ratio (VSWR) at all times. The bandwidth need may be instantaneous and tuning may not suffice.
In the current physics, instantaneous gain bandwidth is linked to antenna size through a relationship known as Chu's Limit (L. J. Chu, “Physical Limitations of Omni-Directional Antennas”, Journal of Applied Physics, Vol. 19, pp 1163-1175 December 1948). Under Chu's Limit, the maximum instantaneous 3 dB gain fractional bandwidth of single tuned antennas may not exceed 200 (r/λ)3, where r is the radius of a spherical envelope placed over the antenna for analysis, and λ is the wavelength. While antenna instantaneous gain bandwidth is limited, voltage standing wave ratio (VSWR) bandwidth is not. Thus, in some systems it may be necessary to trade antenna gain for increased VSWR bandwidth by introducing losses or resistive loading. Losses can be required when the antenna must operate beyond Chu's Limit, that is, to provide low VSWR at small and inadequate sizes. Without dissipative losses, the single tuned instantaneous 2 to 1 VSWR bandwidth of an antenna cannot exceed 70.7 (r/λ)3.
Multiple tuning has been proposed as an approach for extending the instantaneous gain bandwidth of antennas, say with a network external to the antenna, such as impedance compensation circuit. Multiple tuned antennas have polynomial responses and may include rippled passbands like a Chebyshev filter. Although beneficial, multiple tuning cannot be a remedy to all antenna size-bandwidth needs. Wheeler has suggested a 3π bandwidth enhancement limit for infinite order multiple tuning relative single tuning (“The Wideband Matching Area For A Small Antenna”, Harold A. Wheeler, IEEE Transactions on Antennas and Propagation, Vol. AP-31, No. 2, March 1983). Simple antennas may provide a “single tuned” frequency response that is quadratic in nature,
The ½ wave thin wire dipole is an example of a simple antenna. It can have a 3 dB gain bandwidth of 13.5 percent and a 2.0 to 1 VSWR bandwidth of only 4.5 percent. This is near 5 percent of Chu's single tuned gain bandwidth limit and it is often not adequate. Broadband dipoles are an alternative to the wire dipole. These preferably utilize cone radiating elements, rather than thin wires, for radial rather than linear current flow. They are well suited for wave expansion over a broad frequency range. Conical antennas, which include a single inverted cone over a ground plane, and biconical antennas, which include a pair of cones oriented with their apexes pointing toward each other are used as broadband antennas for various applications, such as, for example, spectrum surveillance.
A biconical antenna including a top inverted cone, a bottom cone and a feed structure, is disclosed in U.S. Pat. No. 2,175,252 to Carter entitled “Short Wave Antenna”. Two cones form a self exciting horn which connects to a coaxial circuit that provides an electrical signal that feeds the antenna. The antenna is symmetric about the cone axis and each of the cones is a full cone, spanning 360 degrees. In FIG. 2 of U.S. Pat. No. 2,175,252 a single cone is excited relative a planar member forming a conical monopole. A biconical antenna having for example, a conical flare angle of Π/2 radians has essentially a high pass filter response from a lower cut off frequency. Such an antenna provides wide bandwidth, and a response of 10 or more octaves is achieved. Yet, even conical antennas are not without limitation: the VSWR rises rapidly below the lower cutoff frequency. Low pass response antennas are seemingly unknown in the present art,
Broadband conical dipoles can include dissimilar half elements, such as the combination of a disc and a cone. A discone antenna is disclosed in U.S. Pat. No. 2,368,663 to Kandoian. The discone antenna includes a conical antenna element and a disc antenna element positioned adjacent the apex of the cone. The transmission feed extends through the interior of the cone and is connected to the disc and cone adjacent the apex thereof. A modern discone for military purposes is the model RF-291-AT001 Omnidirectional Tactical Discone Antenna, by Harris Corporation of Melbourne, Fla. It is designed for operation from 100 to 512 MHz and usable beyond 1000 MHz. It has wire cage elements for lightweight and easy of deployment.
U.S. Pat. No. 7,170,462, to Parsche, describes a system of broadband conical dipole configuration for multiple tuning and enhanced pattern bandwidth. Discone antennas and conical monopoles may be related to other by inversion, e.g. one is simply the other upside down. U.S. Pat. Nos. 4,851,859 and 7,286,095 disclose such antennas formed with connectors at the cone and disc, respectively.
Folding in dipole antennas may be attributed to Carter, in U.S. Pat. No. 2,283,914. The thin wire dipole antenna includes a second wire dipole member connected in parallel to form a “fold”. In FIG. 5 of U.S. Pat. No. 2,283,914 the folded dipole member included a resistor for the enhancement of VSWR bandwidth. Without the resistor, bandwidth was not enhanced (relative to an unfolded antenna of the same total envelope) but there were advantages of impedance transformation or otherwise. Resistor “terminated” folded dipoles were employed in World War II. Later, in U.S. Pat. No. 4,423,423 to Bush, a resistive load was described in a folded dipole fold member. Resistively terminated folded wire dipole antennas may have low VSWR but lack sufficient gain away from narrow resonances.
Conventional conical antennas have broad instantaneous bandwidth but rapidly rising VSWR at frequencies below cutoff. To obtain sufficiently low VSWR at low frequencies, they may be too physically large. The large size may cause insufficient pattern beamwidth at the higher frequencies. Accordingly, there is a need for a broadband antenna that provides a low VSWR at many or all radio frequencies, at small size, and that does not suffer from these limitations.